A prior sedimentation system comprising the features of the preamble of the independent device claim is known from DE 44 26 052. This prior device has a rotationally symmetric container comprising an upper cylindrical section and a funnel-shaped section situated below. At the lower end of the funnel-shaped section there is provided a discharge outlet for the removal of sludge which has accumulated in the lower portion of the funnel-shaped section. A pipe is concentrically disposed in the container, which initially tapers downwardly in order to then widen towards the container in the form of a frusto-conical hood. The pipe is vertically slidable with respect to the container, so that the area ratio of the waste water inlet opening formed by the free end of the funnel-shaped hood can be altered with respect to the annular area disposed outside thereof in a radial direction for receiving the ascending flow, provided that the inlet opening is displaced in the area of the funnel-shaped section. By this alteration, the flow conditions are to be adjusted in such a way that the sludge particles descend as fast as possible and the water partly freed from the sludge particles ascends in the container under flow conditions which are favorable for the ongoing separation process, so that residual, usually smaller sludge particles can descend from this ascending flow.